This invention relates to a data reception control device which determines the transmission rate of received data and generates a sampling clock corresponding to the transmission rate for properly receiving the data.
A facsimile modulator/demodulator (hereinafter referred to as FAX modem) is commercially available in recent years that can transmit data from a personal computer to a remote facsimile machine via a telephone line. By connecting the personal computer to the telephone line, the FAX modem adds a capability of facsimile transmission and reception to conventional personal computer communications (hereinafter referred to as PC communications).
The FAX modem is usually provided with an AT command receiver so that its operation is controlled by AT commands. The AT command system uses a set of commands each starting with a combination of characters "AT". Upon receiving the characters "A" and "T", the FAX modem checks data transmission rate and format. A character string following the characters "A" and "T" to constitute a command is then received at the correct data transmission rate and format.
The AT command system includes facsimile-related commands, by which it is possible to select Class 1 communications mode in accordance with Recommendation 578 as stipulated by the Electronic Industries Association (known as EIA) of the United States.
In Class 1 communications mode, transmitting and receiving terminals exchange not only AT commands but also non-AT commands consisting of characters starting with other than "AT" in accordance with a specific protocol. Therefore, the receiving terminal usually comprises an AT command receiving circuit for receiving AT commands as well as a general-purpose receiving circuit for receiving non-AT commands. The receiving circuits need to be switched depending on the type of commands sent from the data transmitting terminal. This construction tends to make receiving circuitry rather complicated.
In a process of data communications in Class 1 communications mode using AT commands, however, non-AT commands are preceded by a predetermined AT command and yet the non-AT commands are transmitted at the same transmission rate as the AT command. Accordingly, provided that the transmission rate of the preceding AT command is determined (regardless of whether the transmission rate of the non-AT commands is determined), the AT command receiving circuit can properly receive the non-AT commands based on the AT command transmission rate.